Pursuant to this, a common view is that the gravitational field itself should, in some way, be quantized. There are several routes to accomplishing this.Indeed, highlighting the interpretational difficulties of QG, is the way that notions of quantum can vary from model to model. A popular class of theories postulates that the combination of quantum theory with gravity should give way to fundamentally discrete models of space and time, such as in the causal set [1]} and causal dynamical triangulation [2]} programs. Most typical, however, is through some manner of quantization algorithm: extending the existing quantum formalism to the gravitational domain by application of the standard ad hoc quantization rules to the appropriate gravitational degrees of freedom. Such is the tack taken by the principal QG candidates — string theory (ST) and loop quantum gravity (LQG).To be sure, although there are vast differences between canonical approaches to QG, including LQG, and ST (see e.g., [3]}), the adoption of a quantization algorithm is possibly one of the few points of intersection. For some historical overviews in the development of ST and LQG, see the reviews by S. Mukhi [4]} and C. Rovelli [5]}, respectively.
